This invention relates to the heat treatment of metals.
One method of heat treatment, in daily commercial use, is the carbonitriding of low carbon steels. If the components to be carbonitrided are of relatively thin cross-section they tend to become physically distorted. Attempts have therefore been made to find alternative heat treatment processes to carbonitriding in order to avoid this problem of distortion. One such process is ferritic nitro-carburising. It has been found that the process of ferritic nitro-carburising of mild steel components enables the problem of distortion to be overcome and enables a scuff-resistant surface finish to be obtained. One drawback of the process of ferritic nitro-carburising is, however, that the components, after treatment by the process, tend to have poor indentation resistance. Another process which overcomes the aforementioned problem of distortion is austenitic nitro-carburising. Moreover, mild steel components can be given good scuff-resistance and indentation-resistance by being subjected to austenitic nitro-carburising.
In order to perform a process of austenitic nitro-carburising of components of mild or other low carbon steel, it has hitherto been the practice to produce the necessary atmosphere by adding ammonia to an atmosphere produced by an endothermic generator and to supply the resultant gas mixture (or the two gas streams separately) to a furnace (typically a sealed quench furnace) in which the treatment is performed. Endothermic generators do however have certain disadvantages. In particular, they can be expensive to maintain, and skilled operatives are often required for this purpose; they are relatively expensive items of capital equipment which have a finite life; and they are quite bulky and take up floorspace which could otherwise be used for a different purpose in a heat treatment workshop.